Metal substrate preparation is a critical process step in the production of commercially suitable YBCO coated conductors. The surface smoothness, uniformity and cleanliness of the metal substrate has a large influence on the texture of the buffer and the current carrying capability of the YBCO superconducting layer deposited on the buffered metal substrate.
Metal substrates up to 100 meters long can be prepared routinely by mechanical/chemical mechanical polishing. Although high current has been achieved in such lengths of mechanically polished YBCO coated conductors, it is difficult to produce conductors with such performance characteristics over long lengths due to defects and deformation generated by contact and the low speed of mechanical polishing.
As stated in Kreiskott, Sascha et al., Continuous electropolishing of Hastelloy substrates for ion-beam assisted deposition of MgO, Supercond. Sci. Technol. 16 (2003) 613–616, commercial applications of high temperature superconducting wires require fast and cost effective production processes. Electropolishing as an alternative approach of substrate preparation has a speed of at least an order of magnitude higher than mechanical polishing. Surface smoothness comparable to mechanical polishing has been achieved in prototype reel-to-reel electro polishing. This invention addresses methods for process control in electro polishing to produce suitable substrates for commercial YBCO coated conductor production.
Theoretically, the uniformity of electropolishing is much better than that of mechanical polishing. Electropolishing can polish metal substrates at speeds in the range of tens of meters per hour. Multiple tapes can be run in parallel or wide tapes may be utilized. Conventional electropolishing processes utilize external mechanical contacts which impose a limit on the scalability of the process. Nonetheless electropolishing appears to be the preparation technique of the future. However, metal tapes thus polished still have microscopic defects that affect the performance of YBCO coated conductors.
Two major defects are observed. The first is the presence of residual particles on the surface of the polished substrate due to residual lubricant on the tape and contamination in the electropolishing process solution and rinsing solutions. The second is the presence of a columnar structure across the tape width due to the vertically circulating whirl flow of the electropolishing solution.
It is important to develop new methods that utilize a modified electropolishing process in order to achieve residue free tapes and to minimize the existence of a columnar structure across the width of the tape.
It is an object of this invention to reduce the defects present on metal substrates to be used to produce superconducting tapes.
It is another object of this invention to greatly increase the performance of YBCO coated conductors.
It is an object of the invention to provide a system and method for surface preparation of long lengths of metal substrates in a continuous manner.
It is another object of the invention to provide a cleaning process for producing a highly clean substrate surface that can, after a subsequent film deposition process, achieve high current densities in long lengths of HTS-coated tape.
It is yet another object of the invention to provide a polishing process for producing a substrate having a smooth surface that is of very high quality in a single polishing pass.
It is yet another object of the invention to provide a substrate for high current density superconducting tapes.